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[FROM THE AMERICAN Journal of SciEnce, Vol. XXXVII, January, 1914]
A MODIFICATION OF THE USUAL METHOD OF
CORRECTING SILICA FOR INCLUDED SALTS
By S. B. KUZIRIAN
(Contributions from the Kent Chemical Laboratory, Yale
University, New Haven, Conn., U. S. A.)
Rºuzirian–Correcting Silica for Included Salts. 61
ART. W.-A Modification of the Usual Method of Correct-
ăng Silica for included Salts; by S. B. KUZIRIAN.
[Contributions from the Kent Chemical Laboratory of Yale Univ.–ccliii.]
WHEN silica, liberated by the action of hydrochloric acid
upon the product of fusion of a silicate with an alkali carbon-
ate, is made insoluble and separated by the usual subsequent
treatment, viz., evaporation, desiccation at 110°, extraction
with dilute hydrochloric acid and water, filtration, and repeated
washing with distilled water, it invariably contains foreign
material which after strong ignition consists essentially of
oxides, free or combined with the silica. If the silica is vola-
tilized by treatment with hydrofluoric acid and sulphuric acid,
the included material is composed of sulphates which remain
as such or are changed to oxides in a subsequent ignition. If
the constituents of this residue were definite and correctly
determinable, it would be possible to calculate the weight of
the oxides as they exist in the strongly ignited silica, and to
make the proper correction ; but it is a difficult task to make a
correct analysis of a residue which sometimes does not exceed
two to three milligrams and may contain iron, aluminum, man-
ganese, titanium, magnesium, sodium, potassium, etc. In order
to avoid making the quantitative analysis of this small residue
of sulphates and oxides, it has been an approved procedure to
blast the silica to constant weight and then to assume that after
the prolonged blast ignition the impurities are left in the form
of oxides. Then, after the removal of the silica with sulphuric
and hydrofluoric acids, the remaining sulphate residue is also
blasted to constant weight, and the assumption is made that
this latter step will transform the sulphates of the residues into
their oxides. The weight of silica is then found by difference.
It will be shown in the following account of experimental work
that the residue obtained after treatment of the blasted silica
with sulphuric and hydrofluoric acids and blasting does not
accurately represent the amount of included material as it is
ordinarily weighed with the silica.
In the presence of moisture from the Bunsen burner or the
blast-lamp, chlorides are slowly decomposed with evolution of
hydrochloric acid and the simultaneous formation of metallic
oxides or, in the case of the alkali metals, hydroxides, which
may then enter into combination with any suitable non-metallic
oxide with which they are in contact. Silica containing traces
of soluble alkali chlorides may, on strong ignition, gradually
combine with the metallic oxides formed, as has been shown in
a previous paper.” The assumption that the impurity in the
silica, contaminated by chlorides when it is separated, con-
sists after the ignition exclusively of oxides is, therefore, well
founded.
* This Journal, xxxvi, 598, 1913.
62 Kuzirian–Correcting Silica for Included Salts.
Alkali sulphates, on the other hand, do not yield definite
and weighable oxides on ignition. Moreover, they may vola-
tilize as such at the high temperature of the blast-lamp. The
experiments of Table I show that the presence of silica does
not materially change this characteristic of alkali sulphates,
and that when a small amount of an alkali chloride is added in
solution to ignited silica, and the mixture evaporated to dry-
ness and treated with sulphuric acid, the residue left after
ignition with the Bunsen burner is essentially the neutral
sulphate.
TABLE I.
Increase in
weight of
silica after
- addition of Error when
Silica Silica Na2SO4 NaCl, treatment the impurity
ignited with ignited with equivalent with H2SO4 and is corrected
Bunsen burner blast-lamp to NaCl. ignition with 8,S
1/2 hour 20 mm. added Bunsen burner Na2SO4
grm. grm. gTrn. grim. glºm.
0.4779 0.477 0-0 122 O'013] + 0.0009
0°5316 0. 5315 O'O 122 ()' 0131 -- 0:0009
O'5266 0°5264 0 0 1 22 0.0122 0' 0000
0 °531 1 ()-5293 0 0 1 15 0 0 1 15 O'0000
That elements other than Sodium and potassium, present orig-
inally with the silica as chlorides and converted to sulphates
by the action of sulphuric acid and gentle ignition, may be
recovered essentially as Sulphates after treatment with sulphuric
and hydrofluoric acids and gentle ignition, is shown in Table II.
TABLE II.
Sulphate
Silica Silica Sulphate determined Sulphate
ignited with ignited with equivalent by increase left after re-
Bunsen burner blast-lamp to chloride in weight moving silica
1/2 hour 20 min. added of silica by H2SO4 + HF
grm. grm. grm. grm. grm.
BaCl2 added.
O'5290 O-5290 0 0 103 ()' 0.098 O'0 104
CaCl2 added.
O'5576 ()'5572 0 0 1 21 0 0 1 14 0-0 1 18
0 °535 1 * * * * * O'02.42 O'02 18 O'0226
()'5486 - - - - - ()' 0242 O'02.42 O'0234.
0°5413 * * * * * 0'02.42 0-0228 0.0234
O'54.86 O'54.86 0 °0242 O'02.42 * * * * *
MgCl2 added
0°5441 O'54.36 O'()200 ():01.95 O'O197
0°5446 - - - - - 0° 0200 0.02 15 0 0220
0°5456 O'5456 ()' 0200 ()'0203 sº sº sº sº, sº
AlCl4 added.
O'5456 gº º sº m, sº 0 0 195 O'0200 O'O | 95
O'5447 tº sº º ºr * ()' 0195 0-019.2 O'0205
0°54.90 0'5.490 0' 019.5 0'0.188 sºme sº * * *
º
Auzirian–Correcting Silica for Included Salts. 63
As is well known, sulphates other than alkali Sulphates, lose
the acidic oxide, more or less according to the conditions,
when submitted to ignition with the blast-lamp. While pro-
longed blast-ignition may bring about a complete transforma-
tion of the sulphates of iron, aluminium, chromium, and
titanium to the respective oxides, the refractory alkali Sulphates,
as well as the sulphates of magnesium, calcium, and barium in
large degree, will remain in the condition of sulphates (though
the alkali sulphates may volatilize appreciably), and the correc-
tion for silica as ordinarily applied will be in error by an
amount approximately equal to that of the sulphur trioxide
combined in the sulphates.
The source of error, inherent in the usual method of apply-
ing corrections to the ignited silica, may be largely avoided by
the introduction of a slight modification of the treatment.
This modification consists essentially in treating the ignited
silica with sulphuric acid, gently igniting again before weigh-
ing, and in igniting under exactly the same conditions the sul-
phate residues left after the removal of the silica in the usual
way. That is to say, the included impurities of the silica must
be transformed before weighing into the condition in which
they will be left when the silica is removed by sulphuric acid
and hydrofluoric acid. This can be accomplished by adding a
few drops of dilute sulphuric acid to the well ignited impure
silica, evaporating the excess of the acid slowly over a radiator
and igniting the residue by means of a Bunsen burner, before
weighing the residue. Then, after the removal of the silica
in the usual manner, the Sulphate residue left is ignited, at the
same temperature and for the same duration of time as was
the silica, and then weighed. From the weight of the silica
plus the impurity before the treatment with sulphuric acid
and hydrofluoric acids and that of the residue after that treat-
ment the weight of the silica is found by difference.
Table III contains the details of experiments in which silica
was fused with six times its weight of sodium carbonate, the
melt treated with hydrochloric acid ; the mixture evaporated ;
the residue desiccated (at 110° (A) or, in presence of acetic
anhydride, at 137° (B)), and extracted as usual with hydro-
chloric acid; the precipitate filtered off and washed ; and the
filtrate treated again like the original solution of the melt for
the recovery of silica soluble in the former operation. The
residues (first, second, and total) and the error which results
from calling these residues oxide instead of sulphate and sub-
tracting their weight from that of the ignited silica, according
to the usual method of making the correction, are shown.
64 Kuzirian—Correcting Silica for Included Salts.
TABLE III.
Error which
|Residue left Residue left results from
- by H2SO4 by H2SO4 calling the
SiO2 + HF treat- + HE treat- residues ox-
(blasted) ment of first ing of second Sum of ide instead
taken precipitate precipitate residues of sulphate
grim. grm. grim. grm. grim.
A. * --
Desiccation at 110°.
0-5.302 0 0010 0’00 l 2 0.0022 + 0°001 3
0-521] 0-0010 0:00 l () 0.0020 -- 0:00 12
O'5440 0’00 l () ()' 0012 0.0022 -- 0:00.13
0 °535.] O' 0010 ()'0002 ()' 00I 2 -- 0-0007
0°5436 ()' 0010 0 °0013 0 0023 + 0-0014
B
Desiccation at 137° in acetic anhydride.
()' 5520 0-0010 O'0003 O ()013 -- 0:000S
0°54.52 0-0010 0-0010 ()' 0.020 + 0-00 || 2
O'5347 0-00 || 0 0'0005 O'00 15 -- 0:0009
()' 552 I 0 0010 0-0005 0 0015 + 0.0009
These errors, which are not inappreciable, were found when
the silica was separated, as well as possible, from sodium
chloride alone. When chlorides of other elements, such as mag-
nesium, calcium, and aluminium, are present the silica sepa-
rated is likely to be also contaminated with these salts which,
by the ordinary treatment, will be transformed to oxides when
the silica is ignited and to sulphates when the silica is
removed; and these sulphates will be more or less refractory
under ignition, according to their natures and to the duration
and temperature of the ignition. The errors shown above for
the case in which sodium chloride is the only contaminating
salt are likely to be magnified in the analysis of ordinary sili-
cates of complex composition. The proposed modification of
treatment will, therefore, lead to a more accurate application
of the correction for impurities included in the silica. In this
procedure it is not necessary to blast either the silica or the
residue before and after the removal of silica as silicon fluoride.
The Bunsen burner will give a temperature sufficiently high to
volatilize the excess of sulphuric acid and to break up the
acidic and pyrosulphates of the alkali elements. The included
Salts being weighed as sulphates, or as sulphates broken up to
practically the same extent in both ignitions, the correction to
be applied will be reasonably accurate.
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